Memory and learning in animals are mediated by neurotransmitters that are released from vesicles clustered at the synapse. As a synapse is used more frequently, its neurotransmission efficiency increases, partly because of increased vesicle clustering in the presynaptic neuron. Vesicle clustering has been believed to result primarily from biochemical signaling processes that require the connectivity of the presynaptic terminal with the cell body, the central nervous system, and the postsynaptic cell. Our in vivo experiments on the embryonic Drosophila nervous system show that vesicle clustering at the neuromuscular presynaptic terminal depends on mechanical tension within the axons. Vesicle clustering vanishes upon severing the axon from the cell body, but is restored when mechanical tension is applied to the severed end of the axon. Clustering increases when intact axons are stretched mechanically by pulling the postsynaptic muscle. Using micro mechanical force sensors, we find that embryonic axons that have formed neuromuscular junctions maintain a rest tension of Ϸ1 nanonewton. If the rest tension is perturbed mechanically, axons restore the rest tension either by relaxing or by contracting over a period of Ϸ15 min. Our results suggest that neuromuscular synapses employ mechanical tension as a signal to modulate vesicle accumulation and synaptic plasticity.T he accumulation of neurotransmitter containing vesicles at the presynaptic terminal is essential for neural communication. On the arrival of an action potential at the terminal, neurotransmitters are released through exocytosis of the vesicles. The transmitters excite the postsynaptic terminal in a millisecond time frame (1). The amount of neurotransmitter release for a given action potential depends on several factors including how frequently the synapse has been used. This usage dependent plasticity is believed to be the basis of memory and learning (2). Despite a wealth of knowledge on the molecular components of the synapse (3, 4) and the modulation of the postsynaptic machinery during long-term potentiation and depression (5-7), the mechanism of vesicle accumulation at the presynaptic terminal and regulation of neurotransmission in a usage-dependent manner remains unclear (8). There is increasing experimental evidence suggesting that the mechanical microenvironment has a significant influence on a variety of cell functions including gene expression, cell growth and morphology, cytoskeletal organization, and apoptosis (9-13). Our in vivo experiments reveal that mechanical tension in axons plays a key role in vesicle accumulation.We examine the neuromuscular synapse within live embryos of Drosophila melanogaster (14) (Fig. 1A). In Drosophila, an aCC (anterior corner cell) pioneer motoneuron extends a single axon and invariably innervates its target, muscle1. A pair of aCC motoneurons occurs in every segment of the embryo, and their development is highly stereotyped. The first contact between the aCC axon and muscle1 occurs at hour 14 of embryogenesis, ...

No abstract

Memory and learning in animals is mediated by neurotransmission at the synaptic junctions (end point of axons). Neurotransmitters are carried by synaptic vesicles which cluster at the junctions, ready to be dispatched for transmission. The more a synapse is used, higher is the clustering, and higher is the neurotransmission efficiency (plasticity), i.e., the junction “remembers” its use in the near past, and modifies accordingly. This usage dependent plasticity offers the basic mechanism of memory and learning. A central dogma in neuroscience is that, clustering is the result of a complex biochemical signaling process. We show, using MEMS sensors and fruit fly (Drosophila) embryo nervous system, that mechanical tension in axons is essential for clustering. Without tension, clustering disappears, but reappears with application of tension. Nature maintains a rest tension of 1nN in axons of Drosophila for learning and memory.